Advanced composite materials are increasingly used in applications such as aerospace structure design due to superior stiffness, strength, fatigue resistance, corrosion resistance, etc. In many instances, the use of advanced composite materials may also greatly reduce the number of parts. Composites present challenges for inspection however due to heterogeneity, anisotropy, and the fact that damage is often subsurface. Despite success in the laboratory setting, many non-destructive testing and monitoring techniques, are impractical for real-world inspection of large-area integrated composite structures, for example, due to the size and complexity of the required support equipment. In addition, many components that need frequent monitoring typically reside in limited access areas that would require breaking of factory seals and calibrations to manually inspect. It is clear that new approaches for inspection are necessary.
To facilitate inspection, a structure may advantageously incorporate a distribution of sensors to provide feedback for or on the structure. Such feedback may include event notifications (such as for impact), structural integrity, usage, shape, and/or configuration. Conventional sensors, however, may add weight to a structure and can present electrical connectivity and mechanical coupling challenges. Conventional sensors often present reliability risks (i.e., many sensors fail and/or malfunction in advance of the structure they are monitoring). Thus structures formed from materials which are capable in themselves of providing feedback are highly desirable. Examples of such integrated feedback materials include but are note limited to materials that change resistance values as they are strained, materials that can provide actuation through phase change, ablative materials, and materials that can store energy.
Carbon nanotubes (CNTs) can posses exceptional mechanical stiffness (e.g., ˜1 TPa) and strength, as well as excellent electrical conductivity (e.g., ˜1000× copper) and piezoresistivity (resistivity change with mechanical strain). Presently, however, it is not possible to produce large specimen (>5 mm) purely of CNTs. Furthermore, due to issues such as agglomeration and poor dispersion, only marginal improvements in mechanical properties are observed for hybrid composites when CNTs are introduced into the bulk matrix. Somewhat better results can be achieved using nanoscale modification of the interface between composite plies, by growing CNTs on the surface of cloth or placing unaligned CNTs at low volume fractions on fibers. However, these approaches do not significantly improve electrical conductivity and thus limit many practical applications.